The optical instrument used to measure and characterize sky quality at the IAC observatories is the DIMM (differential image motion measurements). The optical system and its mode of operation are relatively simple. It consists, basically, placing two equal apertures at the entrance of a telescope, in one of them an optical wedge is located. In this way, two beams of the same object are obtained which will lead to two on the focal plane of the telescope but laterally separated a few seconds arc. The complexity of this optical system lies in the "simplicity" of the plate used to separate the beams, it is a flat-faced wedge of a few minutes, and this is where problems arise when manufacturing it. In this paper we present a new optical system concept to separate the beams. This is done using two optical flats tilted. The optical flats are not placed at the entrance of the telescope, but in the convergent beam. The optical design, manufacture and the test results obtained are presented.